In U.S. Pat. No. 4,907,595 an apparatus for determining the pressure and flow inside the optic artery is described. The apparatus uses a rigid chamber that can be sealingly affixed over a person's eye so that it can be pressurized to apply an external pressure against the eyeball. An ultrasonic transducer is also mounted to the chamber and oriented to inject ultrasonic pulses for a Doppler type measurement of the flow inside the optic artery. The apparatus operates by enabling an operator to increase the pressure to such a level that the blood flow through the optic artery ceases. The pressure at which this occurs is then an indication of the pressure inside the optic artery. Typically the pressure at which this event occurs is in the range of about 170 mm Hg.
A problem associated with an apparatus as described in the '595 patent is that the pressure necessary to obtain the desired measurement is so high that it exceeds generally maximum recommended pressures by a significant amount. When such device is then used for an extended time, tissue damage can arise and may result in an increase in the intracranial pressure, P.sub.IC, to unacceptable levels.
Another ultrasonic device for determining changes in intracranial pressure in a patient's skull is described in U.S. Pat. No. 5,117,835 by placing a pair of ultrasonic transducers against the skull and storing the received signals. U.S Pat. No. 4,984,567 describes an apparatus for measuring the intracranium pressure with an ultrasonic transducer by analyzing the acoustic reflections caused by ultrasonic pulses. Other patents related to ultrasonic measuring of either intracranial pressure or other physiological features are U.S. Pat. Nos. 4,204,547; 4,930,513; 5,0116,641 and 5,040,540.
None of these prior art teachings provide a clear and non-equivocal indication of the intracranial pressure. The measurements tend to be obscured by noise arising from uncertainties in the measurements. There is, therefore, a need to be able to derive a measurement of the intracranial pressure of a person in a safe and a non-invasive manner that can be implemented with reasonable reliability.